Piles are used to prepare the foundation of a structure such as a building, and the depth and load bearing capacity of a pile are critical factors which must be considered and verified when carrying out deep foundation works.
Driven piles and injection piles are both pre-fabricated piles which are usually made of steel, reinforced concrete, wood or a composite of steel and concrete, and differ only in the method which is used to embed the pile into the ground.
A driven pile as the name suggests, is a pile which is driven into the ground mechanically with a pile-driver.
Similarly, an injection pile is a pile which is injected or jacked into the ground mechanically, using a high-capacity hydraulic jacking machine.
Driven piles and injection piles comprise of both the close-ended and the open-ended variety. A close ended driven pile would generally be a steel pipe with a cast steel shoe covering the bottom of the pile, a steel H-beam, or a square or triangular pile with a covered end. An open ended pile, may generally be a steel pipe or tube with an open bottom end, which permits earth to enter the pipe as the pile is driven into the ground.
The verification of the load bearing capacity and the integrity of a pile depends on a number of factors such as ground conditions, availability of test pile data for specific ground conditions and/or specific piling techniques, and involves carrying out both preliminary and/or working pile tests.
For example, the Federation of Piling Specialists Handbook on Pile Load Testing specifies that piling works being carried out on complex or unknown ground conditions, with no previous pile test data available and/or using a new piling technique constitutes a high level of risk, making both preliminary and working pile tests essential, at a rate of one preliminary pile test for every 250 piles, and one working pile test for every 100 piles.
There are a number of load testing methods currently in use, such as the reaction pile method, the kentledge method and the bi-directional load cell method, which all fall under the category of Maintained Load Tests.
Traditionally, the kentledge method is most commonly used for the load testing of driven piles and injection piles. The kentledge method involves equipping a test pile with instrumentation such as dial gauges and load cells to measure displacement, and placing a discrete and incremental load up to a maximum of 120% of the test load on a test frame assembled over a test pile for an extended period of time, and then monitoring and taking measurements of the resulting pile movement and settlement.
However, the kentledge method has a number of drawbacks. The primary one being the test load used in the kentledge method which is generally in the form of concrete blocks or steel ingots of regular dimensions and weight. These concrete blocks are trucked-in to a work-site and then lifted with a crane and stacked incrementally on the test frame. Due to the sheer weight and bulk of the concrete blocks and the rising cost of fuel and transportation, the kentledge method has become increasing expensive and less cost effective. This is particularly true for test loads in the 300 ton range, where the transportation costs increases exponentially for test loads above 300 tons.
Other drawbacks of the kentledge method are:                the requirement for a large working area due to the need for a crane and sufficient area for the delivery and storage of the concrete blocks, and        the inherent risk of collapse of the test frame on complex or unknown ground conditions which poses a danger to workers manning and monitoring the test instrumentation.        
Although there are other Maintained Load Test methods mentioned above such as the bi-directional load test cell which is safer, this method is only applicable for load testing of augered or bored piles, and is not suited for testing of driven piles.
The bi-directional load test method involves placing one or more load cell containing a hydraulic jack either at the pile base, or part way up the bored pile shaft, and then expanding the test cell hydraulically so that the upper part of a bored pile reacts against the lower part of the pile. However, there exists no apparatus to apply the bi-directional load test method to driven piles.
With the drawbacks of the kentledge method in mind, it is desirable to have a test method and apparatus to facilitate the load testing of driven piles which is more economical, and which is safer to operate.
In view of the absence of a comparable bi-directional load test system for driven piles, it is further desirable to provide an apparatus which allows such a load testing method to be applied to driven piles.
The present invention was developed in consideration of the above requirements.